This invention relates to acrylic polymers and more specifically to polyacrylamides and polyacrylates capable of exhibiting nonlinear optical responses in an electric or electromagnetic field. Specifically, this invention is directed to the use of these acrylic polymers as the nonlinear optical component in various electrical devices for purposes of processing optical signals.
Recent developments have led to increased use of optical components in various electrical and optoelectronic devices, i.e. devices containing optical components which are either active or passive components. These devices include, for example, the use of optical materials in second harmonic generators, modulators e.g. spatial light modulators, phase shifters, optical amplifiers, laser technology, interferometers, optical switches, logic gates, communications and computational devices, and alike. It is generally known that various organic compounds and particularly polymeric materials can exhibit nonlinear optical responses which in some instances is larger than some of the inorganic materials.
Nonlinear optics is primarily concerned with the interaction of lightwaves with matter in a way that is not linear in applied and/or optical electromagnetic fields. Current practice utilizes nonlinear crystals such as KDP (potassium dihydrogen phosphate), lithium niobate, etc. for the effects and devices described herein. However, these crystals are difficult and expensive to grow and difficult to use in making optical devices. In addition, these materials exhibit relatively small nonlinear effects and suffer from optical damage at high laser input power. Laser frequency converters, for example, are based generally on inorganic crystals that respond nonlinearly to incident high power optical radiation by changing the frequency of the radiation. Second harmonic generation (SHG) results when optical radiation passes through a transparent medium having an electric susceptibility that is a nonlinear function of the radiation field. In theory, any optically transparent medium without inversion symmetry can produce second harmonic generation provided that the electric field of the electromagnetic radiation is sufficiently large. For example, a light transmitting solid medium should satisfy two structure requirements in order to achieve efficient second harmonic generation. First the optical medium must not be symmetrical about a center point (the nonlinear second order susceptibility vanishes in an optical medium that possesses a centro symmetric structure). Second for maximum second harmonic generation the optical medium must possess a means for making the coherence length large compared to the length of the material, e.g. a propagation direction whereby the optical medium birefringence cancels the natural dispersion leading to a state of equal indices of refraction at the fundamental and second harmonic frequencies.
The advantage of acrylic polymers in comparison to other compounds is that these polymers exhibit high mechanical strength and chemical stability. The addition of an optically nonlinear side-chain to the polymer backbone provides desirable features, e.g. a large nonlinear optical molecular susceptibility in a solid medium. For example, nonlinear optical materials comprised of polymerized aromatic compounds are disclosed in U.S. Pat. No. 4,431,263 where the theoretical principles of nonlinear behavior of organic systems are discussed. U.S. Pat. No. 4,199,698 discloses the use of a single crystal of 2-methyl-4-nitroaniline in nonlinear devices. Further, U.S. Pat. No. 4,748,074 discloses nonlinear optical compositions comprising copolymers with an additive having a molecular optoelectronic activity e.g. methyl-N-(2,4-dinitro phenyl)alaninate. Thus, there is a continuous effort to develop new nonlinear optical polymeric systems for devices such as laser modulation and deflection, information control in optical circuitry, light valves and various types of optical switches, etc.
While certain organic molecules are known to exhibit extremely large optical nonlinearities, it was not until recently that these materials have been used in electro-optic devices. The materials primarily used for such optic devices include the inorganic crystals, e.g., lithium niobate which has a reasonably high electro-optic coefficient; they are used primarily because large crystals can be manufactured with low defect densities. Although there are other organic and inorganic materials which are more effective than lithium niobate, these materials are based on second harmonic generation (SHG) tests of materials in the aqueous or powder form. It is essential that the material have a uniform large solid form before it can be considered for electro-optic uses. One of the advantages of utilizing acrylic polymers is because of the ability to alter the molecular structure and thereby optimize the nonlinear optical and physical properties of the polymers. This ability to synthesize specific polymeric structures makes the material particularly useful for the fabrication of various electro-optic devices.